# The upper limit for the atomic adsorption energy value on a slab surface?

I want to adsorb atomic oxygen on a slab surface.

The adsorption energy I am calculating is around -10 eV. Which seems higher to me than I expected.

My question is this: Is this value reasonable or is there some issue in my calculation?

To address the question in the title, I went to Catalysis-Hub and sorted the computed energies from most exothermic to most endothermic. Ignoring the C (g) entries adsorbing onto surfaces (which have computed adsorption energies around -7 eV), the most exothermic entry is $$\mathrm{I_{2}\ (g)} + 2* \rightarrow 2 \mathrm{I}^*$$ on a Pt(111) surface with a value of -3.49 eV with the SCAN functional. This definitely seems very exothermic to me.

Anyway, for what you're actually asking, -10 eV sounds far too exothermic. It is almost certainly unreasonable. Perhaps your surface structure rearranged upon adsorption, there is some algebra error, some settings were changed, or a spin state was not correctly accounted for.

• Is I_{2} meant to be $I_2$ ? Apr 30 '20 at 21:20
• Yup, adjusted now. Apr 30 '20 at 21:22
• What about the * for the 2? Apr 30 '20 at 21:24
• No, that should not be superscripted because it's considered a species. It should be superscript on the I^* though to indicate the phase. IMO. Apr 30 '20 at 21:24
• Okay, I was just asking, because the original version had a superscript :) Apr 30 '20 at 21:25

I would just like to point out that the question explicitly mentions adsorbing atomic oxygen rather than $$\mathrm{O_{2}}$$ (without mentioning the oxygen source or the surface).

One can therefore somewhat simplify the question to: what is the strongest bond the oxygen atom can form?

In molecules, the strongest bond is the C≡O triple bond in carbon monoxide at 11.16 eV (experimental value at room temperature).

I.e. adsorbing atomic oxygen on a hypothetical "atomic carbon substrate" would indeed give an energy in the 10 eV range. For realistic substrates like amorphous carbon, however, where surface carbons are bound to other carbons, the energy of the oxygen double bond should be significantly lower (I guess more in the 7-8 eV range).